The present invention is directed, in general, to voltage-controlled dimming illuminated displays and, more specifically, to voltage-controlled dimming of multi-color displays illuminated by light emitting diodes.
Commercial and military aircraft crewstation instrumentation displays, like many other display systems, frequently employ illuminated indicators and controls. Traditionally, incandescent lamps operating at 5 VAC, 14 VDC or 28 VDC have been employed as illumination sources for illuminated pushbutton switches, indicators and annunciators within aircraft instrumentation. The illumination from such incandescent lamps is generally optically filtered to produce a wide range of human visible or night vision imaging system (NVIS) colors, including blue, green, yellow, red and white or the NVIS colors of NVIS blue, NVIS green A (blue-green), NVIS green B (green), NVIS yellow (yellow) and NVIS red (orange-red). In addition, the small size of incandescent lamps allows multiple lamps to be used within the same display to illuminate different regions of the display in different colors, providing, for example, four separate legends, each in a different color, on the same display for a single illuminated pushbutton switch, indicator or annunciator.
Color filters for incandescent displays usually comprise acrylic, polycarbonate, or glass materials, or some combination thereof. The filters are employed to adjust the basically Planckian spectral radiance of the incandescent lamps to meet the desired chromaticity coordinates and luminance levels at the maximum rated voltage(s) for the lamps. By combining multiple lamps and different color filters, displays can have multiple legends each lighting in different colors.
The filters employed for incandescent lamp displays also adjust the luminance of the display. The luminance of incandescent displays is approximately 400 foot-lamberts at full rated voltage for sunlight-readability in daytime flying. Such luminance levels are too bright for night flying, requiring the display luminance to be reduced for dark adaptations to prevent loss of visibility of outside imagery. The display luminance is typically reduced to 15 foot-lamberts for commercial/general aviation night flying, 1.0 foot-lambert for military night flying, and 0.1 foot-lamberts for night flying utilizing NVIS night vision goggles.
Because the luminance of incandescent lamps varies with applied voltage within a certain range, the voltage supplied to the displays is reduced to approximately one-half or less of the normal full rated operating voltage to reduce luminance levels for night flying conditions. Because all incandescent lamps have similar electrical and optical characteristics, each display and display color can maintain a luminance uniformity (of average luminance for different colors at a given voltage) of 4:1 or better as the voltage is reduced.
Unfortunately, however, the inherent characteristics of incandescent lamps lead to noticeable chromaticity shifts as the applied voltage is reduced. Moreover, incandescent lamps suffer other disadvantages when employed in aircraft instrumentation, including high power consumption, high inrush current, uncomfortably high touch temperatures, and unreliability in high vibration environments. As a result, considerable effort has been expended to incorporate more stable, efficient and reliable technologies, such as light emitting diodes (LEDs), into aircraft crewstation illuminated displays.
Light emitting diodes with narrow band spectral radiance are commercially available in a wide variety of colors suitable for aircraft crewstation illumination, including red, yellow, green and blue. Multiple light emitting diodes each emitting in a different color allow displays to have legends which light in different colorsxe2x80x94that is, green legends illuminated by green light emitting diodes, yellow legends by yellow light emitting diodes, etc. Separate legends on a single display may light in different colors in this manner.
Using simple series resistance driver circuits tailored to the characteristics of the light emitting diodes, displays constructed for single color light emitting diodes are capable of producing sufficient luminance to be considered sunlight readable. While such displays function adequately at the maximum rated voltage for the particular light emitting diodes employed, problems arise when the applied voltage is reduced on the displays for night flying conditions. Due to differences in the bandgap voltage and voltage-luminance characteristics, different color light emitting diodes dim at different rates as the voltage is reduced to reduce the luminance of the display. As a result, light emitting diodes of one color extinguish before light emitting diodes of other colors have satisfactorily dimmed. Therefore, one colored legend on a particular display may become so dim as to be unreadable while an adjacent legend in a different color remains too bright for night flying.
The problems associated with voltage-controlled dimming of different color light emitting diodes arise from the inverse relationship of the bandgap voltage, and therefore the forward voltage drop, of a light emitting diode to the peak emission wavelength. For example, red light emitting diodes emitting at a wavelength of approximately 620-650 nanometers (nm) typically begin emitting at roughly 1.6 VDC, while yellow light emitting diodes emitting at a wavelength of about 570-590 nm begin emitting at approximately 1.8 VDC, green light emitting diodes emitting at a wavelength of about 520-540 nm begin emitting at approximately 2.2 VDC, and blue light emitting diodes emitting at a wavelength of about 470-480 nm begin emitting at approximately 2.5 VDC. Using voltage control techniques to reduce the luminance of multi-color displays, where each illumination color is provided by a different color light emitting diode, results in drastically different dimming for different color light emitting diodes. As the voltage is reduced, blue and green light emitting diodes tend to dim much faster than yellow and red light emitting diodes, often extinguishing before a voltage is reached which produces acceptable night flying luminance levels for the yellow and red light emitting diodes.
As a result, uniform luminance control of different color light emitting diodes has not been previously achieved with voltage dimming. Instead, dimming of displays using different color light emitting diodes has been through the use of pulse width modulation (PWM) or constant current drivers. The complexity of constant current circuitry and the high level of electromagnetic interference inherent to pulse width modulation have slowed integration of light emitting diodes into aircraft crewstation illumination.
There is, therefore, a need in the art for uniform voltage-controlled dimming of multi-color legends within a single display.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in aircraft crewstation displays, multi-color illumination for a single display, which in the present invention is achieved by utilizing an array of white light emitting diodes to produce illumination optically filtered by separate filters to produce the two or more desired output illumination colors at a desired luminance when the full rated voltage is applied to the light emitting diodes. Since all of the light emitting diodes have the same bandgap, voltage-controlled dimming produces uniform luminance changes for all colors as the voltage is reduced. A single voltage-controlled dimming driver circuit design and a single control voltage may therefore be utilized to achieve desired and uniform dimming characteristics independent of output illumination color.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9corxe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.